1. Field of the Invention
The present invention generally relates to optical monitoring of manufacturing processes and, more particularly, to the optical inspection of apertures in a substrate or lamina.
2. Description of the Prior Art
In order to obtain good quality during the course of a manufacturing process, it is a generally accepted practice to inspect the results of most, if not all, operations. Such inspections are particularly necessary where automated equipment is used to perform the process. If an operation is carried out improperly, causing defects in the workpiece which cannot be remedied, further operations on the defective workpiece can be avoided. If the defect can be remedied, appropriate action can be taken at the most advantageous point during the further manufacturing operations.
Such inspections are particularly necessary where automated equipment is used to perform the process. Inspection of the workpiece after an operation is carried out can also reveal much important information concerning the operating condition of the automated apparatus such as wear, alignment and other adjustments.
Recent designs of electronic components are generally characterized by the small feature size of patterns which may form or interconnect many different elements of the electronic component although many features of the pattern may be formed during a single manufacturing operation. Due to the large numbers of features which may result from a single manufacturing operation, automated inspection systems have been developed to reduce inspection times. Exemplary automated inspection systems are discussed in U.S. Pat. Nos. 4,040,745 to J. G. Belleson and 4,464,050 to K. Kato et al. Other exemplary arrangements are disclosed by L. P. Hayes in "Scanner for Opaque Samples", IBM Tech. Discl. Bull. Volume 16, No. 7, (February, 1972), and "Laser Scan System for Surface Inspection", IBM Tech. Discl. Bull. Vol. 30, No. 11 (April, 1988).
Virtually all such inspection systems relying on reflection of radiation from a surface share the common features of a source of radiation, a means for focussing that radiation on an inspection spot and a detector for radiation transmitted through or reflected by the sample. Such systems may include other optical elements such as filters to improve the signal to noise ratio of the observation or elements such as mirrors or beam splitters to provide scanning or to improve the measurement geometry of the system.
However, it is well-accepted that the resolution of such systems is limited by the size of the focussed spot of radiation. That is, a defect which is smaller than the area of the focussed spot will cause only a small difference in reflection or transmission and may not be detected. U.S. Pat. No. 4,999,510 to F. Hayano et al varies the beam size to discriminate foreign particles of different sizes. Further, in manufacturing processes where features are formed on a surface, such as through-holes, generally referred to as vias (particularly when filled with a conductive material), in an insulating lamina or substrate, an improperly formed or missing feature may have similar reflectance to the surrounding surface. It is also likely that the size of a defect in a feature will be smaller than the feature itself. Therefore, expensive and complex optical systems have been employed to reduce the size of the illuminated radiation spot in order to improve resolution of optical inspection systems.
Increases in resolution of the inspection systems present additional problems since it results in additional data to be processed. The data processing for feature extraction is computationally intensive and the finer segmentation of the image in accordance with finer resolution increases the data to be processed by the square of the increase in linear resolution (e.g. lines per mm.). Therefore, as the trend in electronic circuit modules has increased and minimum feature size has decreased, dictating finer resolution from automated inspection systems, the computational loads required for analysis of the inspection data has limited throughput of the manufacturing processes as well as greatly increased the expense of the inspection systems. The limitation of the manufacturing processes also increases the cost of the components manufactured.